Connected: An Internet Encyclopedia
RFC 793 - GLOSSARY

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### GLOSSARY

    1822
              BBN Report 1822, "The Specification of the Interconnection of
              a Host and an IMP".  The specification of interface between a
              host and the ARPANET.

    ACK
              A control bit (acknowledge) occupying no sequence space, which
              indicates that the acknowledgment field of this segment
              specifies the next sequence number the sender of this segment
              is expecting to receive, hence acknowledging receipt of all
              previous sequence numbers.

    ARPANET message
              The unit of transmission between a host and an IMP in the
              ARPANET.  The maximum size is about 1012 octets (8096 bits).

    ARPANET packet
              A unit of transmission used internally in the ARPANET between
              IMPs.  The maximum size is about 126 octets (1008 bits).

    connection
              A logical communication path identified by a pair of sockets.

    datagram
              A message sent in a packet switched computer communications
              network.

    Destination Address
              The destination address, usually the network and host
              identifiers.

    FIN
              A control bit (finis) occupying one sequence number, which
              indicates that the sender will send no more data or control
              occupying sequence space.

    fragment
              A portion of a logical unit of data, in particular an internet
              fragment is a portion of an internet datagram.

    FTP
              A file transfer protocol.

    header
              Control information at the beginning of a message, segment,
              fragment, packet or block of data.

    host
              A computer.  In particular a source or destination of messages
              from the point of view of the communication network.

    Identification
              An Internet Protocol field.  This identifying value assigned
              by the sender aids in assembling the fragments of a datagram.

    IMP
              The Interface Message Processor, the packet switch of the
              ARPANET.

    internet address
              A source or destination address specific to the host level.

    internet datagram
              The unit of data exchanged between an internet module and the
              higher level protocol together with the internet header.

    internet fragment
              A portion of the data of an internet datagram with an internet
              header.

    IP
              Internet Protocol.

    IRS
              The Initial Receive Sequence number.  The first sequence
              number used by the sender on a connection.

    ISN
              The Initial Sequence Number.  The first sequence number used
              on a connection, (either ISS or IRS).  Selected on a clock
              based procedure.

    ISS
              The Initial Send Sequence number.  The first sequence number
              used by the sender on a connection.

    leader
              Control information at the beginning of a message or block of
              data.  In particular, in the ARPANET, the control information
              on an ARPANET message at the host-IMP interface.

    left sequence
              This is the next sequence number to be acknowledged by the
              data receiving TCP (or the lowest currently unacknowledged
              sequence number) and is sometimes referred to as the left edge
              of the send window.

    local packet
              The unit of transmission within a local network.

    module
              An implementation, usually in software, of a protocol or other
              procedure.

    MSL
              Maximum Segment Lifetime, the time a TCP segment can exist in
              the internetwork system.  Arbitrarily defined to be 2 minutes.

    octet
              An eight bit byte.

    Options
              An Option field may contain several options, and each option
              may be several octets in length.  The options are used
              primarily in testing situations; for example, to carry
              timestamps.  Both the Internet Protocol and TCP provide for
              options fields.

    packet
              A package of data with a header which may or may not be
              logically complete.  More often a physical packaging than a
              logical packaging of data.

    port
              The portion of a socket that specifies which logical input or
              output channel of a process is associated with the data.

    process
              A program in execution.  A source or destination of data from
              the point of view of the TCP or other host-to-host protocol.

    PUSH
              A control bit occupying no sequence space, indicating that
              this segment contains data that must be pushed through to the
              receiving user.

    RCV.NXT
              receive next sequence number

    RCV.UP
              receive urgent pointer

    RCV.WND
              receive window

    receive next sequence number
              This is the next sequence number the local TCP is expecting to
              receive.

    receive window
              This represents the sequence numbers the local (receiving) TCP
              is willing to receive.  Thus, the local TCP considers that
              segments overlapping the range RCV.NXT to
              RCV.NXT + RCV.WND - 1 carry acceptable data or control.
              Segments containing sequence numbers entirely outside of this
              range are considered duplicates and discarded.

    RST
              A control bit (reset), occupying no sequence space, indicating
              that the receiver should delete the connection without further
              interaction.  The receiver can determine, based on the
              sequence number and acknowledgment fields of the incoming
              segment, whether it should honor the reset command or ignore
              it.  In no case does receipt of a segment containing RST give
              rise to a RST in response.

    RTP
              Real Time Protocol:  A host-to-host protocol for communication
              of time critical information.

    SEG.ACK
              segment acknowledgment

    SEG.LEN
              segment length

    SEG.PRC
              segment precedence value

    SEG.SEQ
              segment sequence

    SEG.UP
              segment urgent pointer field

    SEG.WND
              segment window field

    segment
              A logical unit of data, in particular a TCP segment is the
              unit of data transfered between a pair of TCP modules.

    segment acknowledgment
              The sequence number in the acknowledgment field of the
              arriving segment.

    segment length
              The amount of sequence number space occupied by a segment,
              including any controls which occupy sequence space.

    segment sequence
              The number in the sequence field of the arriving segment.

    send sequence
              This is the next sequence number the local (sending) TCP will
              use on the connection.  It is initially selected from an
              initial sequence number curve (ISN) and is incremented for
              each octet of data or sequenced control transmitted.

    send window
              This represents the sequence numbers which the remote
              (receiving) TCP is willing to receive.  It is the value of the
              window field specified in segments from the remote (data
              receiving) TCP.  The range of new sequence numbers which may
              be emitted by a TCP lies between SND.NXT and
              SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers
              between SND.UNA and SND.NXT are expected, of course.)

    SND.NXT
              send sequence

    SND.UNA
              left sequence

    SND.UP
              send urgent pointer

    SND.WL1
              segment sequence number at last window update

    SND.WL2
              segment acknowledgment number at last window update

    SND.WND
              send window

    socket
              An address which specifically includes a port identifier, that
              is, the concatenation of an Internet Address with a TCP port.

    Source Address
              The source address, usually the network and host identifiers.

    SYN
              A control bit in the incoming segment, occupying one sequence
              number, used at the initiation of a connection, to indicate
              where the sequence numbering will start.

    TCB
              Transmission control block, the data structure that records
              the state of a connection.

    TCB.PRC
              The precedence of the connection.

    TCP
              Transmission Control Protocol:  A host-to-host protocol for
              reliable communication in internetwork environments.

    TOS
              Type of Service, an Internet Protocol field.

    Type of Service
              An Internet Protocol field which indicates the type of service
              for this internet fragment.

    URG
              A control bit (urgent), occupying no sequence space, used to
              indicate that the receiving user should be notified to do
              urgent processing as long as there is data to be consumed with
              sequence numbers less than the value indicated in the urgent
              pointer.

    urgent pointer
              A control field meaningful only when the URG bit is on.  This
              field communicates the value of the urgent pointer which
              indicates the data octet associated with the sending user's
              urgent call.

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Connected: An Internet Encyclopedia
RFC 793 - GLOSSARY